An injection laser is a semiconductor device which normally acts as an efficient and compact source of coherent light. Use is being made of such devices in data communications and in recording, printing and displaying apparatus.
In order to be operated satisfactorily in most applications, it is necessary that an injection laser be supplied with a bias current. Examples of use of a fixed value bias current for an injection laser may be found, for example, in U.S. Pat. Nos. 3,319,080, 3,815,045, 3,925,735, 3,968,399, and 4,027,179.
Feedback control of laser bias current is also known. If the bias current is controlled by a negative feedback system responsive to the light output, then there is compensation for differences in individual device characteristics and for time varying changes in device characteristics due, for example, to aging or temperature changes.
Examples of the use of feedback control of bias current for an injection laser may be found in U.S. Pat. Nos. 3,946,335 and 4,009,385 and in the following technical articles: J. Gruber et al, "Electronic Circuits for High Bit Rate Digital Fiber Optic Communications Systems", COM-26 IEEE Trans. Comm. 1088-1098 (July 1978); D. Smith, "Laser Level-control Circuit for High-bit-rate Systems Using a Slope Detector", 14 Elect. Lett. 775-6 (1978); P. Schumate et al, "GaAlAs Laser Transmitter for Lightwave Transmission Systems", 57 Bell Sys. Tech. Jour. 1823-1836 (1978).
All of these feedback systems are of the analog type, even when the injection laser modulation signal is digital in nature. This is apparent because there is no conversion of the sensed light amplitude into a digital code, and the bias current (as opposed to the modulation signal) for the injection laser is not produced by converting a digital representation into analog form. That the bias current is analog in character is also apparent from the continuous nature of the amplitude value it can assume. If the bias current were produced as a result of conversion from a digital code, the bias current could assume only discrete values in the steady state (after transients due to a change from one discrete value to another have disappeared). Thus, the prior art has used analog feedback to control the bias level of an injection laser even though the signals superimposed on the controlled bias level usually are digital in nature.
The prior art circuits are inherently difficult to monolithically integrate, since they include circuit elements such as capacitors and inductors which are inherently difficult to integrate.
These circuit elements are needed in such analog circuits because of the need to have one or more low pass networks. Such networks are used, for example, to prevent the modulation signal from adversely affecting the negative feedback bias stabilization system and in the feedback system itself to perform operations such as mathematical integration. These prior art circuits also provide no extended or flexible set of control or test functions since each additional function requires additional circuitry. As a result, the prior art control circuits lack versatility. Different modes of operation and/or different applications usually require a different control circuit.
Monolithic integration offers cost and performance advantages over other methods of implementation. It also makes the size of the control system sufficiently small that it can be placed in the same module as the injection laser and associated output optical fiber. An extended set of control and test functions, furthermore, offers the flexibility in application which is generally associated with digital circuit elements.
It is an object of this invention to provide a bias current control circuit for an injection laser which can be readily integrated monolithically.
A further object is to provide a control circuit for an injection laser which has an extended and flexible set of control, modulation, and test functions.
Another object is to provide an injection laser control circuit which can be manufactured and tested on a conventional manufacturing line for digital components.
Still another object is to provide an injection laser control circuit in a size sufficiently small that it can be packaged in the same module as the injection laser and associated output optical fiber.
It is also an object to provide a digital control circuit for an injection laser.